6.3.1 Carbon Dioxide

IPCC (1990) and the SAR used a radiative forcing of 4.37 Wm-2 for
a doubling of CO2 calculated with a simplified expression. Since
then several studies, including some using GCMs (Mitchell and Johns, 1997; Ramaswamy
and Chen, 1997b; Hansen et al., 1998), have calculated a lower radiative forcing
due to CO2 (Pinnock et al., 1995; Roehl et al., 1995; Myhre and Stordal,
1997; Myhre et al., 1998b; Jain et al., 2000). The newer estimates of radiative
forcing due to a doubling of CO2 are between 3.5 and 4.1 Wm-2
with the relevant species and various overlaps between greenhouse gases included.
The lower forcing in the cited newer studies is due to an accounting of the
stratospheric temperature adjustment which was not properly taken into account
in the simplified expression used in IPCC (1990) and the SAR (Myhre et al.,
1998b). In Myhre et al. (1998b) and Jain et al. (2000), the short-wave forcing
due to CO2 is also included, an effect not taken into account in
the SAR. The short-wave effect results in a negative forcing contribution for
the surface-troposphere system owing to the extra absorption due to CO2
in the stratosphere; however, this effect is relatively small compared to the
total radiative forcing (< 5%).

The new best estimate based on the published results for the radiative forcing
due to a doubling of CO2 is 3.7 Wm-2, which is a reduction
of 15% compared to the SAR. The forcing since pre-industrial times in the SAR
was estimated to be 1.56 Wm-2; this is now altered to 1.46 Wm-2
in accordance with the discussion above. The overall decrease of about 6% (from
1.56 to 1.46) accounts for the above effect and also accounts for the increase
in CO2 concentration since the time period considered in the SAR
(the latter effect, by itself, yields an increase in the forcing of about 10%).

While an updating of the simplified expressions to account for the stratospheric
adjustment becomes necessary for radiative forcing estimates, it is noted that
GCM simulations of CO2-induced climate effects already account for
this physical effect implicitly (see also Chapter 9). In some climate
studies, the sum of the non-CO2 well-mixed greenhouse gases forcing
is represented by that due to an equivalent amount of CO2. Because
the CO2 forcing in the SAR was higher than the new estimate, the
use of the equivalent CO2 concept would underestimate the impact
of the non-CO2 well-mixed gases, if the IPCC values of radiative
forcing were used in the scaling operation.